This is why there can be different versions of the same song, but one can be be considered "the" great one/defining version.

That's not quite true though. There have been a few cases, especially in the '60s, where a song became a regional hit in the US or in another country and then another artist covered it and their version became a national hit. The only difference being that one label was exploiting the other label's lack of distribution. For example, Shadows of Knight had a hit with Gloria because the version by Them was censored by some radio stations because Van Morrison said "she comes up to my room, she makes me feel alright."